1. Technical Field of the Invention
The present invention relates to a method and apparatus for measuring thickness variation of a thin sheet material. More particularly, the present invention relates to measurement of thickness variation of wafers for use in semiconductor devices, which need to meet stringent requirements for little variation in thickness in a surface direction, and further relates to a probe reflector used in the thickness variation measuring apparatus.
2. Description of Related Art
Wafers used for semiconductor devices consist of a thin sheet material of silicone or the like. For fabrication of semiconductor elements and circuits on the surface of a wafer, photoetching techniques, printing techniques, and various micro machining techniques are applied.
In processing a wafer using these techniques, it is essential to achieve high planarity of the wafer surface. Deterioration in surface planarity may blur the patterns of semiconductor elements or circuits formed by photoetching, or make the contour of a material printed in patterns on the wafer surface unclear. With the increase in density and scale of semiconductor elements and circuits, the above problem becomes crucial.
In the fabrication of semiconductor devices, it is the normal practice to hold the entire surface of the wafer on a flat support surface by means of vacuum suction or the like during various processes. If the thickness of the wafer varies in different locations, the planarity of the wafer surface will in turn vary, as the backside of the wafer is held on a flat support surface in tight contact therewith. There is thus the requirement for minimization of variation in thickness of the wafer. Evaluation of wafer thickness variation in a wafer production process entails precise and efficient measurement of variation in wafer thickness.
Japanese Laid-open Patent Application No. 10-70162 discloses an apparatus for measuring thickness variation of a wafer. In this technique, a disk-shaped wafer is rotated as held vertically, and using a capacitance displacement sensor disposed opposite both faces on one side of the wafer, thickness variation of the wafer is calculated from the measured displacement of wafer surface with respect to the sensor. The capacitance displacement sensor is scanned in a radial direction of the wafer, thereby measuring the thickness variation of the entire surface of the wafer.
However, the above described apparatus has a limit to measurement precision, and is not suitable for highly precise thickness variation measurement required for the fabrication of semiconductor devices of high density in recent years.
The capacitance displacement sensor used in the above described apparatus measures displacement of wafer surface with respect to the sensor by electric measurement of capacitance between the wafer surface and the sensor. For that reason, the measurement results are highly dependent on electrical properties of the wafer which may change depending on the material of the wafer or ambient conditions. Such electrical properties may also vary in different locations on a same wafer. Measurement precision of wafer thickness variation by this capacitance sensor is thus apt to deteriorate. Further, precise measurement of capacitance at the peripheral edge of the wafer is practically impossible, and since the thickness variation in this part cannot be estimated, it is normally regarded that such peripheral portion of the wafer to a width of about 3 mm from the outermost edge of the wafer cannot be used for semiconductor devices. Wafer material is accordingly wasted. Moreover, in some cases depending on conductive properties of the wafer, measurement by the above capacitance displacement sensor is technically difficult. Also, as noted above, measurement results are greatly affected by ambient conditions between the sensor and the wafer.
In accordance with the increase in density of the semiconductor elements and circuits in recent years, it is the normal requirement that thickness variation measurement of a wafer should be carried out with the precision lower than 0.01 xcexcm.
Theoretically, such high precision in thickness variation measurement can hardly be achieved with the above described capacitance displacement sensor, which is said to have the precision of about 0.05 xcexcm.
Apart from wafers for semiconductor devices, there is a technical field where high precision is required in the measurement of thickness variation of a thin sheet material such as a substrate for magnetic disks.
In view of the foregoing, an object of the present invention is to provide a method and apparatus for measuring thickness variation of a thin sheet material such as a wafer highly accurately and efficiently.
In accordance with the present invention, using a pair of optical displacement gauges, measuring light is irradiated onto a surface of the thin sheet material, and displacement of surface position of the thin sheet material is measured by receiving said measuring light reflected by the surface of the thin sheet material. Thickness variation of the thin sheet material is thereby obtained from the measured displacement of the surface position of the thin sheet material with the optical displacement gauges.
The present invention can be applied to any type of thin sheet material irrespective of material, configuration, and dimension, as long as measurement of thickness variation thereof with high accuracy is required. Material may be both conductive and non-conductive. Also, the thin sheet material may be constructed such that characteristics or electric properties thereof vary at different locations, or may be constructed multi-layered with a plurality of different materials. Specifically, thin sheet material includes wafers for semiconductor devices made of silicone or the like, metal plates for magnetic disks, ceramic plates, resin plates, and others. The shape of the thin sheet material is mostly disk-like or circular shape particularly in the case of wafers, but it is not limited to the circular shape.
The surface of the thin sheet material should preferably have superior reflectance such as a mirror plate. However, in the case of using the probe reflector of the present invention to be described later, the surface of the thin sheet material need not particularly have superior reflectance.
The apparatus for measuring thickness variation of a thin sheet material according to the present invention comprises a pair of optical displacement gauges disposed opposite each other, with the thin sheet material being arranged between the two optical displacement gauges. The thin sheet material is supported such as to be rotatable, so that measurement by the optical displacement gauges can be carried out at different locations along a circumferential direction. Furthermore, the optical displacement gauges are movable in a direction along radius of rotation of the thin sheet material, so that, combined with the rotation of the thin sheet material, measurement can be made with respect to the entire surface of the thin sheet material. It should be noted that the optical displacement gauges themselves need not be moved, but an optical system may be provided, which changes position of irradiating measuring light to the thin sheet material and position of receiving the reflected measuring light, for achieving the same function. Such scanning measurement is especially suitable for quality control in a production line.
By summing up the amounts of displacement of the surface positions of both faces of the thin sheet material that are measured respectively by the two optical displacement gauges, thickness variation of the thin sheet material can be obtained. An electronic circuit will suffice for performing such calculation for obtaining thickness variation of the thin sheet material. Alternatively, an operation/processing device such as a microcomputer may be used, in which predetermined operating and data processing procedures are programmed in advance.
For the optical displacement gauge, any type of gauge or measuring device can be used, that has a function of measuring the distance or change in the distance to an object being measured by irradiating measuring light to the object and by receiving the reflected measuring light, in order to determine displacement of the surface position of the object.
Specifically, various techniques that utilize the theory of triangulation, optical interferometry, or holography, are known. For the measuring light, the method of using a single wavelength light and the method of using a plurality of wavelengths light can both be applied. Also, an optical three-dimensional shape measuring apparatus, or a shape recognizing sensor can be utilized.
According to the present invention, the optical displacement gauge comprises a light output section for generating light consisting of reference light and measuring light; a light separation and gathering section for separating said light output from the light output section into the measuring light and the reference light, said measuring light being irradiated onto the surface of the thin sheet material, and for gathering the measuring light reflected by the surface of the thin sheet material and the reference light together; and a light receiving and calculation section for receiving the measuring light and the reference light gathered in the light separation and gathering section, and for calculating displacement in the surface position of the thin sheet material.
While the optical path of the measuring light varies depending on the distance from the optical displacement gauge to the surface of the thin sheet material, the optical path of the reference light remains constant. Accordingly, by measuring the difference in optical path of the measuring light and the reference light, displacement in the surface position of the thin sheet material can be obtained. Such difference in the optical path of the measuring light and the reference light can be readily detected in the light receiving and calculation section by making the wavelength of the measuring light and the reference light output from the light output section different.
In the light output section, the reference light and measuring light are generated by a laser oscillator or the like, with which the wavelengths of these light beams are precisely controlled. The light separation and gathering section comprises an optical system including a polarizing beam splitter, a xcex/4-wave plate, and mirrors. The light receiving and calculation section comprises photoelectric converters, electric circuits for processing electric signals, operation circuits, and others.
Further, a converging lens is disposed between the light output section and the light separation and gathering section for converging the light output from the light output section into the light separation and gathering section. The converging lens focuses the measuring light irradiated onto the thin sheet material so that the measuring light is directed only in a limited area on the thin sheet material, thereby enhancing measurement precision. This converging lens is disposed not between the light separation and gathering section and the thin sheet material, but between the light output section and the light separation and gathering section according to the present invention, whereby the optical path of the measuring light from the light separation and gathering section to the thin sheet material is shortened, and adverse effects of scattering caused by aerosol present between the optical displacement gauge and the thin sheet material can be reduced.
Further, a converging optical system is disposed between the light separation and gathering section and the light receiving and calculation section for converging the light output from the light separation and gathering section into the light receiving and calculation section. The converging optical system comprises various optical members such as lenses and mirrors. This converging optical system causes the composite light consisting of measuring light and reference light to be irradiated precisely onto a receiving surface in the light receiving and calculation section, thereby enhancing measurement precision. The measuring light reflected on the surface of the thin sheet material is normally inclined or deviated with respect to the optical passage into the light receiving and calculation section because of an inclination on the surface of the thin sheet material. The provision of the converging optical system ensures that the reflected measuring light is precisely irradiated onto the light receiving surface in the light receiving and calculation section even when the measuring light is inclined or deviated after being reflected by the surface of the thin sheet material.
A probe reflector used in the optical displacement gauge according to the present invention comprises: a base end fixedly mounted to a body of the optical displacement gauge; a free end including a probe that is brought in contact with the surface to be measured and a reflection surface that reflects the measuring light irradiated thereonto; and a support arm for connecting said free end to the base end, comprising a pair of plate pieces that can be elastically deformed, said plate pieces being spaced from each other in a direction orthogonal to the surface to be measured and arranged parallel to each other.
Such probe reflector can be applied broadly to an optical measuring device for obtaining position information of a surface to be measured by receiving measuring light reflected by the surface being measured.
While novel features of the invention are set forth in the preceding, the invention, both as to organization and content, can be further understood and appreciated, along with other objects and features thereof, from the following detailed description and examples when taken in conjunction with the attached drawings.